Magnesium, a light metal, is an attractive material used in constructions, for example, in the automobile and space industries, for manufacturing of cases for notebooks, mobile phones, etc. However, it has rather a low level of strength, toughness and plasticity, caused by the h.c.p. crystal structure. In addition, magnesium has a low resistance to corrosion because of its strong chemical activity. Thus, the only way to use magnesium in some industrial fields is to create magnesium-based alloys with improved properties.
The influence of alloying elements on mechanical and corrosion properties of magnesium alloys is well studied in binary systems, but in multi component alloys their mutual (namely: combined, joint, aggregate, etc.) influence can appear complex and unpredictable. Therefore, the choice of the basic alloying elements and their interrelation in an alloy has defining influence on its properties.
Industrial alloys of magnesium are subdivided into groups according to additional alloying elements such as lithium, aluminium, zinc, yttrium, etc. For example, under the ASTM specification there are groups of magnesium alloys based on lithium—LA (Mg—Li—Al), LAE (Mg—Li—Al—P3M), aluminium—AM (Mg—Al—Mn), AZ (Mg—Al—Zn), AE (Mg—Al-RE), where RE stands for rare earth metals, based on zinc—ZK (Mg—Zn—Zr), ZE (Mg—Zn-RE) and ZH (Mg—Zn—Th); or on the basis of yttrium—WE (Mg—Y—Nd—Zr), etc.
Many patents describe alloys that have more complex compositions and which cannot be clearly assigned to any class under the ASTM specification. The basic aim for the development of these alloys is improvement of certain properties of magnesium that can be used in various technical fields. Mechanical properties of magnesium alloys as well as other metallic alloys with fixed composition are operated by changing the worked combination of hardening and plastic deformation mechanisms. The latter can be modified, in turn, as due to change of a structural condition of an alloy and so also due to a use of special heat treatments.
Corrosion rate of magnesium strongly depends on its purity. For example, in a 4% water solution of sodium chloride a corrosion rate of magnesium purity of 99.9% is hundreds of times higher than for magnesium with purity of 99.99%.
Alloying elements of an alloy, their distribution as well as the composition of the chemical compounds that they form also influences the resistance to corrosion. The corrosion rate of magnesium alloys depends on the structural condition of an alloy and the methods of manufacturing it. In addition, some impurities can change the requirements for a tolerance range of other alloying elements. So, some introduction of aluminium into a magnesium based alloy can increase an influence of other alloying elements on the corrosion rate of an alloy.
Alloys of the present invention are intended to be used mainly at room temperature and for applications demanding good formability and high corrosion stability. Therefore previous developments regarding improvement of mechanical and corrosion properties of magnesium alloys will be considered below under the specified temperature conditions. The data on improvement of strength-, creep resistance- and corrosion characteristics of magnesium alloys at elevated and high temperatures will be considered only partially. This data will be dropped, because, though the improved strength of such alloys will be maintained at room temperatures, the plastic characteristics in these conditions can be strongly reduced.
Unless specified otherwise, the description of the properties of the known magnesium alloys will relate to the range of temperatures varying from 20-50° C., and the composition of the alloys will always be defined as a percentage on weight. (Note: The definition “percentage on weight” is used most often, but “mass percentage” is truer from the physical point of view, because a body weight is different at different geographical breadths of the Globe, and body mass is constant. Our compositions, we show result below is in “mass percentage”.
Mg—Li alloys are the most plastic alloys of magnesium, but their main problem is low corrosion stability and strength. For example, at a room temperature the ultimate elongation of alloy Mg-11% Li reaches 39% at the strength of 104 MPa (see U.S. Pat. No. 2005/6 838 049). However, the corrosion rate of Mg—Li alloys is rather high even in the pure water.
Mg—Li alloys are additionally doped to increase their strength and corrosion stability. Most often aluminium and zinc are added to the alloy to increase strength and corrosion stability. The addition of aluminium and zinc (4% and 2% respectively) leads to a satisfactory combination of strength and deformability of Mg—Li—Al—Zn alloys. It is shown that the addition of 0.6% Al into the alloy Mg-9% Li leads to substantial increase in strength at temperatures below 200° C. in a wide range of deformation rates. Corrosion stability of alloys with such composition increases also.
Some other combinations of alloying elements are available for alloys Mg—Li system. U.S. Pat. No. 2005/6 838 049 describes “Room-temperature-formable magnesium alloy with an excellent corrosion resistance”. Its composition includes from 8.0 to 11.0% lithium, from 0.1 to 4.0% zinc, from 0.1 to 4.5% barium, from 0.1 to 0.5% Al, and from 0.1 to 2.5% Ln (the total sum of one or more lanthanides) and from 0.1 to 1.2% Ca, the balance being Mg and inevitable impurities (the balance was not made with magnesium, it was taken as base (or consisted of Mg and inevitable impurities) and alloying elements were added to it). The invention puts emphasis on precipitation of the phase Mg17Ba2 (Mg17Ba2 is a chemical combination named in crystallography as “phase”), providing refinement and uniform dispersion of an alpha- and beta-phases of alloy matrix. Such structure raises the strength of an alloy. However, though barium has b.c.c. lattice, it has a low solubility limit in Mg and forms intermetallic compounds Mg17Ba2 that noticeably reduce plastic characteristics of Mg—Li alloys.
U.S. Pat. No. 1991/5 059 390 describes “a dual-phase magnesium-based alloy consisting essentially of about 7-12% lithium, about 2-6% aluminum, about 0.1-2% rare earth metal, preferably scandium, up to about 2% zinc and up to about 1% manganese. The alloy exhibits improved combinations of strength, formability and/or corrosion resistance”.
JP Pat. No. 1997/9 241 778 discloses a magnesium alloy being used as a construction material, containing up to 40% Li and one more additive from the following: up to 10% Al, up to 4% Zn, up to 4% Y, up to 4% Ag and up to 4% RE.
In U.S. Pat. No. 1993/5 238 646 the method of preparation of an alloy having an improved combination of strength, formability and corrosion resistance is described. The specified alloy includes 7-12% lithium, 2-7% aluminium, 0.4-2% rare earth metal, up to 2% zinc and up to 1% manganese, the balance being magnesium and impurities. Purity of the magnesium taken for a basis of an alloy is 99.99%.
Mg—Al alloys are most widespread class of magnesium alloys for various applications (groups: AM, AZ, AE etc.). However, though they show raised corrosion resistance and have higher strength, they are much less plastic than Mg—Li alloys. Various combinations of alloying elements are offered for improvement of the certain properties of this class of alloys.
U.S. Pat. No. 2005/0 129 564 describes an alloy consisting of 10 to 15% Al, 0.5 to 10% Sn, 0.1 to 3% Y and 0.1 to 1% Mn, the balance being Mg and inevitable impurities. The magnesium alloy exhibits good creep properties and is particularly suitable for engine related parts.
U.S. Pat. No. 2002/6 395 224 describes an alloy which “includes magnesium as a main component, boron of 0.005 weight % or more, manganese of 0.03 to 1 weight %, and substantially no zirconium or titanium. This magnesium alloy may further include aluminum of 1 to 30 weight % Al and/or zinc of 0.1 to 20 weight %. Because of appropriate amounts of boron and manganese contained in the magnesium alloy, the grain of the magnesium alloy is refined.” The structure refinement leads to increased mechanical characteristics of this alloy.
In U.S. Pat. No. 2005/0 095 166 is disclosed a heat resistant magnesium alloy for casting that includes 6-12% aluminum, 0.05-4% calcium, 0.5-4% rare earth elements, 0.05-0.50% manganese, 0.1-14% tin, balance are magnesium and inevitable impurities. The problem of this invention is the improvement of heat resistance for the magnesium alloy.
Among Mg—Zn alloys the mostly known alloys are: ZK (magnesium-zinc-zirconium) having good strength and plasticity at a room temperature, ZE (magnesium-zinc-RE) having average strength and ZH (magnesium-zinc-thorium) having high room-temperature yield strength in the aged condition (T5). However, alloys containing thorium are not manufactured anymore because of their radioactive components.
U.S. Pat. No. 2001/6 193 817 describes another magnesium based alloy for high pressure die casting (HPDC), providing good creep and corrosion resistance. The alloy comprises at least 91 weight percent magnesium, 0.1 to 2 weight percent of zinc, 2.1 to 5 percent of a rare earth metal component and 0 to 1 weight percent calcium.
However, Al and Zn and some other alloying elements improve strength and corrosion characteristics of Mg alloys and simultaneously reduce their plasticity. In addition, these elements are unsuitable for using alloys in structural elements of endoprosthesises (not biocompatible).
Among Mg-RE alloys compositions of the WE type (Mg—Y—Nd—Zr) are the most known. These alloys have quite a good formability and increased corrosion resistance. According to the specification of the Manufacturer (Magnesium Elektron Ltd., Manchester, England) the ultimate elongation for alloy ELEKTRON WE43 can reach 16% at a room temperature, and the corrosion rate is equal 0.1-0.2 mg/cm2/day (B117 salt spray test) or 0.1 mg/cm2/day (sea water immersion test). However, in many cases deformability of alloy WE43 is insufficient, and the spread of mechanical characteristics for ingots is very great: the elongation varies from 2-17%, in average 7%, data of the Manufacturer for 215 samples. When deformed and treated for stabilization and age-hardening (condition T6), alloys WE43 show more stable, but still low plasticity at a room temperature—up to 10%.
Various changes of a composition of Mg-RE alloys are offered how to increase their characteristics. U.S. Pat. No. 2003/0 129 074 describes high temperature resistant magnesium alloys containing at least 92% magnesium, 2.7 to 3.3% neodymium, 0.0 to 2.6% yttrium, 0.2 to 0.8% zirconium, 0.2 to 0.8% zinc, 0.03 to 0.25% calcium and 0.00 to 0.001% beryllium. The alloy may additionally contain up to 0.007% iron, up to 0.002% nickel, up to 0.003% copper and up to 0.01% silicon and incidental impurities.
Corrosion stability of any magnesium alloys lowers inversely with the Fe, Ni and Cu impurity levels. According to the prior art, alloy AZ91E has a corrosion rate in salt fog tests 100 times lower than alloy AZ91C, due to the higher purity of its alloy basis (0.015% Cu, 0.001% Ni, 0.005% Fe, the others of 0.3% max—in the alloy AZ91E, and 0.1% Cu, 0.01% Ni, the others of 0.3% max—in the alloy AZ91C).
The JP Pat. No. 2000/282 165 describes an Mg—Li alloy with the improved corrosion resistance. The alloy contains up to 10.5% Li and magnesium with a concentration of iron<=50 p.p.m., which is provided by a fusion in a crucible that is covered by chrome and its oxide.
During the last decade interest appeared on the magnesium alloys as material suitable for construction of vascular (coronary and peripheral) endoprosthesises (stents).
Stents are implanted into a vessel lumen after carrying out a percutaneous transluminal coronary angioplasty (PTCA), while the narrowed (stenosis) vessel lumen is expanded by means of an inflated balloon, after the balloon has been positioned at the affected site of the vessel. Stents in form of scaffolds prevent the collapse of the expanded vessel and provide the necessary blood stream through the lumen.
One of the side effects of angioplasty is the phenomenon called restenosis, a rapid proliferation of smooth muscle cells inside the vessel lumen caused by the injury of PTCA. The smooth muscle cells proliferation lasts generally 1-3 weeks. This effect is currently prevented by the use of stents coated with drugs, such as sirolimus or paclitaxel. Unfortunately, because cell proliferation is sometimes prevented too efficiently, the metallic surface of the stent may remain uncoated for months and may provoke the occurrence of coronary thrombosis, sometimes months or years after the coated stent has been implanted in the artery. This may lead to sudden death, sometimes many years after the stent implantation.
As aforesaid, many researchers are interested in biosoluble, biodegradable, or bioresorbable stents. The important advantage of such stents consists in a slow dissolution in vivo of the stent structural material and in gradual disappearance of this device after it has executed its medical function of supporting the vessel wall. In such a way, the disappearance of the stent will avoid the occurrence of thrombosis formation.
Stent materials should have particular mechanical characteristics in order to withstand the elastic recoil due to the vessel wall pressure (radial stability) and to increase the initial stent diameter (for example, under an action of balloon pressure) up to the working size without destruction of stent struts. Besides, the material of stents should be biocompatible, free of harmful impurities, and should not elute toxic substances during degradation in vivo (see U.S. Pat. No. 2005/0 246 041).
Some of the known biosoluble stents are made from various organic polymers having very low mechanical characteristics. These stents are bulky and temperature-sensitive.
Most perspective materials for the manufacturing of biodegradable stents are metallic alloys which may be dissolved in liquids and tissue of a living body (in vivo). Magnesium alloys have been explored for this purpose.
DE Patent No. 2002/10 128 100 describes a medical implant made from magnesium alloy containing additions of rare earth metals and lithium with the following preferred features: 0-7 wt. % lithium, 0-16 wt. % aluminum and 0-8 wt. % rare earth metals. The rare earth metals are cerium, neodymium and/or praseodymium. Examples of alloys are Mg Li4 Al4 SE2 (where SE=rare earth) or MgY4SE3Li2.4. This patent describes experiments on animals too, with stents made from the AE21 alloy and which efficiency is evaluated.
US Pat. No. 2004/0 241 036 discloses further medical implant for the human or animal body made from an alloy that consists at least partially of a magnesium alloy. The magnesium alloy contains portions of rare earth metals and lithium and optionally yttrium and aluminum. The magnesium alloy preferably contains lithium in a portion of 0.01 to 7 mass %, aluminum in a portion of 0.01 to 16 mass %, optionally yttrium in a portion of 0.01 to 7 mass % and rare earth metals in a portion of 0.01 to 8 mass %.
US Pat. No. 2004/0 098 108 describes endoprostheses with a carrier structure, which contains a metallic material, wherein the metallic material contains a magnesium alloy of the following composition: magnesium>90%, yttrium 3.7%-5.5%, rare earths 1.5%-4.4% and balance<1%. This composition corresponds essentially to the alloy WE43.
Other patents of the same inventors (EP 2004 1 419 793, WO 2004 043 474, EP 2005/1 562 565, US 2005/0 266 041, US 2006/0 052 864, EP 2006/1 632 255, US 2006/0 246 107) are variants of the initial document, the DE Pat. No. 10 (2) 53 634.1, priority date Nov. 13, 2002. They carry different names (“Endoprosthesis”, “Endoprosthesis with a supporting structure of magnesium alloy”, “Use of one or more elements from the group containing yttrium, neodymium and zirconium”, “Implant for vessel ligature” etc.) and various items in the claims (time of dissolution in vivo, medical efficiency of alloy components), but all have one common subject, i.e. stents made of the type WE43 alloy.
The search for a suitable material is complicated and expensive (US Pat. No. 2005/0 266 041). All previously known solutions have hitherto not led to a satisfactory result. Apparently, from this point of view, the aforementioned group has been chosen for stent manufacturing the industrial alloy WE43 provides a good (for magnesium alloys) combination of corrosion stability and plasticity.
However, the WE43 alloy is apparently not optimal as a constructional material for manufacturing of biosoluble stents (insufficient plasticity and corrosion stability in vivo). As a proof of this impression one may have a look at the last patents of the specified inventors—the US Pat. No 2006/0 052 863. A wide variation of concentration of the basic alloying elements is patented in it: Y: 2-20%, RE: 2-30%, Zr: 0.5-5.0%, balance: 0-10%, Mg—up to 100%. It is particular to emphasize that the alloying set still coincides with a set of the WE43 alloy.
The document “Peng et al: “Microstructures and tensile properties of Mg-8Gd-0.6Zr-xNd-yY (x+y=3, mass %) alloys” Materials Science And Engineering A: structural Materials: Properties, Microstructure & Processing, Lausanne, CH, vol. 433, no. 1-2, 15 Oct. 2006 (2006-10-15), pages 133-138, XP005623386 ISSN: 0921-5093″ discloses the alloy Mg-8Gd-0.6Zr-2Nd-1Y (page 133, column 2, alloy (B); Nd being a rare earth metal) having a fine grain size of 60-120 um (p. 134, col. 2, end).
The mechanical characteristics and corrosions rates of some most widely known magnesium-based alloys are summarized in the table 1 (data are taken from different sources).
TABLE 1Comparative characteristics of some magnesium alloysUltimateCorrosion rateYS,UTS,elongation,(conditionALLOYMPaMPa%unknown)ConditionWE43*195280100.1 mg/cm2/dayExtruded,(sea waterT5immersion)0.1-0.2 mg/cm2/dayASTM B 117 saltspray testWE4318030010—Forging,T5WE43190270162.5 mg/cm2/Extruded,day**T4AZ 91D160230 3<0.13 mg/cm2/Cast, FdayASTM B 117 saltspray testAM 60B1302206-8<0.13 mg/cm2/Cast, FdayASTM B 177 saltspray testAZ 6123031016Deformed, FZK 6029536012Deformed,T5AM 160130220 8Cast, FMg—11Li—10539Cast, FAlloy of215290251.1**Deformed,invention,H2example 1Alloy of190275291.8**Deformed,invention,H2example 2*Letters in names of alloys designate: A—aluminium, E—the rare earth metals (RE), K—zirconium, L—lithium, M—manganese, W—yttrium, Z—zinc; and figures - the maintenance of an alloying element approximated to an integer in percentage.**The tests for corrosion were led by a special technique. Corrosion rate was calculated after staying of specimens in a stream of 0.9% sodium chloride solution at a flow rate of 50 m/minute. Corrosion rate was defined on the loss of specimen weight and by quantity of the magnesium, which excreted into the solution. Data of measurements were averaged. Such test scheme allows continuously deleting the product of corrosion from sample surface, which deform results of the corrosion rate measurement by method of sample weight loss measurement.